1. Field of the Invention
This invention relates generally to a transfer case for an all-wheel drive (AWD) vehicle and, more particularly, to a transfer case for an AWD vehicle, where the transfer case includes a center differential and a synchronizer to allow shifting between all-wheel drive and two-wheel drive at any vehicle speed.
2. Discussion of the Related Art
Various sport utility vehicles (SUV), off-road vehicles, four wheel drive vehicles, etc. are equipped with drive modes that allow the vehicle to be driven in one or more of two-wheel drive high, four-wheel drive high, four-wheel drive low and AWD. Typically, these types of vehicles employ transfer cases that distribute the drive power received from an output of the vehicle""s transmission to a pair of drive shafts. One of the drive shafts drives the vehicle""s front wheels, and the other of the drive shafts drives the vehicle""s rear wheels. Known transfer cases have employed various types of couplings, such as viscous couplings, electromagnetic clutches, positionable spur gears, etc., that allow the drive power from the transmission to be distributed to the two output shafts of the transfer case to provide the two-wheel drive high, four-wheel drive high, four-wheel high drive low and AWD.
First generation transfer cases employed couplings that could only shift between the various drive modes when the vehicle was stopped. Typically, an adjustable coupling was used to manually shift between drive modes using a mechanical shift actuator. Modern transfer cases sometimes employ synchronizers to synchronize the speed of the input and output shafts of the transfer case prior to shifting between drive modes to allow drive mode shifts while the vehicle is moving.
Various synchronizers have been proposed in the art to provide input and output shaft synchronization of the transfer case. For example, it is known to employ a clutch that translates torque either directly between the input and output shafts of the transfer case, or between the input and output shafts through a planetary gear set. Clutches of this type may be spring-biased, which does not allow a full shift until the relative speeds of the rotating members to be coupled have achieved a certain level of synchronization. However, these known synchronizers typically are complex and add excessive costs to the transfer case. Other transfer cases that employ mechanical shift actuators sometimes fail to provide adequate synchronization prior to the shift, resulting in slip and/or unacceptable shift noises.
In modern transfer cases, mechanical shift actuators have been replaced with electronically controlled shift actuators that are typically operated by an electric motor. In one known transfer case of this type, a reversible DC electric motor is employed to rotate a cammed shift actuator to selectively shift drive gears within the transfer case. A desired drive mode is selected by operating the motor under the control of a microprocessor-based control circuit. The microprocessor commands a motor drive circuit to energize the motor to run in either the clockwise or counter-clockwise direction to achieve the desired drive mode.
In another electronically controlled shift actuator for a transfer case, sensors are employed to sense the speed of the input and output shafts of the transfer case. A microprocessor measures the change in speed over time of each of the input and output shafts, and makes a prediction when the relative speeds of the shafts will be equal. The microprocessor controls a shift actuator at a predetermined time before the shafts are synchronized so that shifting is accomplished when the speeds of the input and output shafts are substantially equal.
In accordance with the teachings of the present invention, a vehicle transfer case is disclosed that allows shifting between AWD and two-wheel drive at any vehicle speed. The transfer case includes a center differential assembly employing pinion and sun gears that allow the front drive shaft and the rear drive shaft to rotate at different speeds to provide the all-wheel drive function. The transfer case further includes an electrically actuated synchronizer assembly that allows the speed bf the front drive shaft to be synchronized to the speed of the rear drive shaft when shifting from two-wheel drive to all-wheel drive to provide the shift on the fly function. The synchronizer assembly also includes a neutral position where both the front drive shaft and the rear drive shaft are not engaged to the vehicle transmission to allow for vehicle towing.
Additional objects, advantages and features of the present invention will become apparent to those skilled in the art from the following discussion and the accompanying drawings and claims.